In combustion chambers for gas turbines, it is an aim to reduce the emissions, such as nitrogen oxides NOx and/or carbon monoxide CO. The temperatures inside the combustion chamber cause high CO and NOx emissions.
In order to reduce the emissions, so-called dry low emissions (DLE) combustion systems are used which typically have a supplementary fuel stream which burns in a diffusion flame mode. The supplementary fuel stream may generate a so-called pilot flame. This stabilizes a main flame in the combustion chamber. In order to reduce the emissions, the pilot flame has to be limited.
Moreover, it is an aim to reduce the temperatures in the combustion chamber in order to decrease the NOx emissions. These lower temperatures may be achieved by burning a lean fuel air mixture in the main flame. A lean fuel air mixture comprises more air than is required to fully burn all the fuel. The lean fuel air mixture therefore comprises a lower flame temperature. However, lean flames can be very unstable and only small changes in the equivalence ratio (the ratio of the fuel/air mix to the fuel/air mix required for all the oxygen and all of the fuel to be completely used in combustion, known as stoichiometric mix) can cause a flameout.
The dry low emission combustion systems comprise a main flame where a very lean fuel mixture is burned. Under normal circumstances this would cause an unstable flame which is prone to dynamics. Therefore, the DLE systems use the pilot flame. A pilot flame comprises a rich or richer fuel mixture, wherein the rich or richer pilot flame is more stable than the lean main flame and the heat and radicals produced from this hot stable pilot frame stabilizes the main flame.
However, the pilot flame is locally hot and therefore causes high NOx emissions.
In order to stabilize the main flame in a combustion chamber, further energy sources may be used to stabilize the main flame.
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